The present invention relates to a method for slicing a single crystal of semiconductor silicon into wafers. More particularly, the invention relates to a method for slicing a single crystal of semiconductor silicon into wafers not by a mechanical means but by a chemical means.
As is well known, single crystals of semiconductor silicon are used almost without exception in the form of a thin wafer having a thickness of, for example, 0.6 mm or smaller. Silicon wafers are obtained conventionally by slicing a single crystal by a mechanical means. For example, a so-called internally bladed slicing machine equipped with a wheel, which is a thin plate of stainless steel in an annular form having a thickness of a few hundreds micrometers, of which fine diamond particles of 40 to 60 .mu.m diameter as an abrasive are electrodeposited on and embedded in the internal periphery of the annular plate to form a cutting blade, and the single crystal of silicon are put under an adequate contacting force at the diamond blade of the annular plate rotating at a velocity of several thousands rpm under tension in the radial direction so that the diamond particles grind off the single crystal material by the thickness of the blade finally resulting in cutting through of the single crystal to give wafers by repeating this cutting procedure.
The above described mechanical method for slicing a silicon single crystal has several problems and economical disadvantages. For example, firstly, it is unavoidable that a material loss caused in the single crystal in every slicing movement corresponding to the thickness of the cutting blade so that the yield of the wafers obtained by slicing is limited by the thickness of the cutting blade which is usually several tenths of the wafer thickness. Secondly, such a mechanical means of cutting naturally has a serious influence on the surface of the wafer to give a large stress thereto as sliced so that the surface layer of the wafer having a thickness of 15 .mu.m or larger is under a strong stress not to exhibit the properties inherent in the single crystal per se so that the wafer is unavoidably warped by 10 to 50 .mu.m. Accordingly, subsequent finishing works are indispensable including lapping to improve the flatness of the wafer by means of rolling abrasive particles and chemical etching to remove the stressed surface layer. An additional amount of the material loss is again caused by these finishing works sometimes amounting 150 .mu.m or even larger in thickness as a total per wafer. Thus, it is not rare that the overall material loss calculated as thicknesses of the wafer is 400 .mu.m or larger per single wafer having thickness of 0.45 mm or smaller.
With an object to decrease the material loss in the preparation of silicon wafers by slicing single crystals, a so-called "wire-sawing" method has been recently proposed in which a fine metal wire serves as a cutting blade in place of conventional diamond-bladed wheels. This method, however, is also a mechanical cutting method so that, even though the material loss by slicing could be reduced to some extent, no remedy can be obtained relative to the stressed surface layer and deformation of the silicon wafers formed by the mechanical cutting means.